The principle: To preach the Gospel. You may say that's obvious but most sermons I hear tell me either a self-help platitude, fail to mention Jesus and the Cross or tell me to try harder and do a few good works. None of these are good news to me nor to anyone else. Jesus is Good News, he has completed his salvation work for humanity on the Cross, it is finished and we can live in power and life from this truth and out of this truth by the power of his Spirit.
It would be easy to divide this parable (as with so many) into a religious message. The good servants and the bad servants with central conclusion that we must go on the coffee rota at church to improve our rating. That is not the Gospel. So where is it.
vs 32 'Do not be afraid'
The Lord has and will provide everything for you. Your possessions and gifts are his and they are for you to steward. You are to use them in a ministry to feed and bless and serve others.
Why does he expect so much of his disciples? Because he has done much. More than 'much' he has done everything. Where you have failed to do his will (as you and I often do), he has not and he has received the many blows that are yours and mine. " They mocked and beat him " Luke 22:63 tells us. He took the beating. His grace is sufficient. His price is enough. We serve and love and provide for others out of his strength, his resources, his life and his resurrection. That is the Gospel. Not our sacrifice but his.
Scott McKnight is helpful in his eight marks of a robust gospel and here are some useful gospel definitions.